This invention relates to overhead projection systems.
Typically, overhead projectors comprise a housing or lamp box which includes therein the necessary light source and condensing lens system. A transparency is normally positioned atop the box and light is passed therethrough to a projection lens which projects the transparency's image on a screen. The most commonly used condensing lens is a Fresnel, which is basically a series of concentric simple lenses. Understandably, the function of the Fresnel is to concentrate the light from the light source at the entrance pupil of the projection lens. Alternative forms of projection may involve using a reflective condensing lens arrangement but the most preferred, for practical reasons including tolerances and uniformity, is the refractive condenser.
When projecting large transparencies, it was necessary in the past to utilize a "fast" condensing lens since projection design goals include minimizing the height of the projection system, including both the light box and the projector head located thereabove. The problem inherent in "fast" condensing lenses is the resulting dispersion which is induced by a relatively large edge ray deviation. Unless the projection lens pupil is increased to accommodate this dispersion, color can appear in areas of the projected image over some portions of the focusing range due to selective vignetting. Accordingly, the aforementioned problems lead to a desire for decreasing the speed of the condensing lens. Such a decrease could improve illumination uniformity over the projective field by reducing the inherent condensing lens losses which increase toward the field's edge.
One of the most recent attempts to decrease the condensing lens speed in an overhead projector is described in U.S. Pat. No. 3,770,344 (Fukushima). In this arrangement, four lamps are utilized under a composite Fresnel lens assembly in order that each lamp is conjugate to the projection lens. One particular problem of this system is that the human eye is very sensitive to contiguous luminance differences and unless all four lamps and respective lensing members are perfectly balanced in terms of luminous output, the four sections of the resulting image will appear as four distinctly different individual areas. Subsequent lamp replacement may also create an imbalance in the system of U.S. Pat. No. 3,770,344. Additionally, misalignment of a single lamp can produce asymmetry in the system with the result being a variation in the respective quadrant of the image.
It is believed therefore that an overhead projection system which overcomes the aforementioned difficulties of prior art systems would constitute a significant advancement in the art.